A hybrid vehicle includes therein an engine producing a driving force with combustion of a fuel, and a motor producing a driving force with supply of electric power. The hybrid vehicle is arranged to select one of an electric vehicle (EV) mode of driving the vehicle with the motor only, by holding the engine inoperative and a hybrid vehicle (HEV) mode of driving the vehicle with the engine and motor, to start the vehicle when the engine water temperature is higher than a predetermined temperature (−10° C.˜−20° C., for example), and to select an engine start mode to start the engine and thereafter start the motor, to start the vehicle when the engine water temperature is lower than or equal to the predetermined temperature. In the EV mode and HEV mode, the motor is operated as a generator for generating electric power according to the situation.
To start the engine at such a lower temperature, a low temperature time hydraulic, or hydraulic control technique is proposed in patent document 1 by the applicant of the present application. This low temperature time hydraulic control technique is a technique to alleviate the drive load of the oil pump due to a low viscosity of the operating oil of the transmission at the time of low temperature start or cold start, by holding the line pressure for operating the automatic transmission at a low level (as an option, simultaneously decreasing the engine rotational speed), and thereafter increasing the command pressure immediately to a greatest value.
However, the low temperature time hydraulic control technique of the above-mentioned patent document 1 when applied to a hybrid vehicle causes following problems.
The above-mentioned low temperature time hydraulic control is started when the system detects an engine start at a low temperature at which the engine water temperature is lower than or equal to the predetermined temperature, and performed to limit the line pressure of the transmission to a small value for a predetermined time period and thereafter increase immediately to a greatest pressure of the command value.
In the case of the hybrid vehicle, however, in the situation in which the engine water temperature is higher than the predetermined temperature, the engine is started, after a start of the motor, by the driving force of the motor, and moreover the time required for starting the engine is short, whereas, in the situation in which the engine water temperature is lower than the predetermined temperature, the engine is started first by the starter, and the time required for starting the engine is long. Therefore, if the system is arranged to start the low temperature time hydraulic control from the start of the engine at a low temperature and terminate the low temperature time hydraulic control at the expiration of a predetermined time period, then the system might terminate the low temperature time hydraulic control too early and cause stoppage of the oil pump due to the overwhelming torque of the oil pump driven by the motor in the latter case while the former case is not problematical.